This invention relates to cold isostatic processing of compactible materials.
Die-compaction is the dominant method of pressing powder materials into complex shapes of accurate dimensions. However, this method is limited to forming parts of small to moderate cross-sectional area. The obtainable compaction is generally in two directions along a single axis, and suffers from frictional drag between the die and powder particles which can result in non-uniform densities within the pressed material. These non-uniformities in density become more evident as the axial thickness increases. Also, as cross-section of the part to be produced increases a greater total pressure is required for compaction. Such tonnage requirements become a practical limitation on this process, and die-compaction is therefore commonly referred to as "force-limited".
Isostatic pressing is another process for forming of components from particulate materials. In cold isostatic pressing (CIP), a powder charge is loaded into an elastomeric mold (called a "bag"). The bag can be considered as a hermetically sealed pressure transfer membrane. The bag may be part of the pressurization (containment) vessel (dry bag process) or may be a separate unit inserted into the pressurization vessel (wet bag process). In either case, a mandrel may be included within the bag to aid in forming details of the pressed material.
The sealed flexible bag is sealed within the Pressurization vessel and is then exposed to a pressurized fluid environment to promote material consolidation/compaction. In operation, the fluid is pressurized and this in turn applies a hydrostatic pressure to the loaded bag. The bag transfers the fluid pressure to the powder. This isostatically compresses the powder charge, at ambient temperatures, within the pressurization vessel. If a mandrel is included inside the bag, then the pressure compacts the powder against the mandrel
Upon completion of the CIP process, the pressure is relieved and the pressure vessel is unsealed. The bag is then removed and opened. The part (called a "compact") is separated from the bag and mandrel. The surface formed by the elastomeric bag leaves a mottled surface on the compact, while a smooth or detailed surface is left by the mandrel according to the surface finish on the mandrel. This process results in near net shaping of components with generally uniform as-pressed densities The compact is then thermally treated, i.e., sintered, to increase its strength through diffusion bonding, and is machined to net shape as required.
Isostatic pressing is generally considered to be non-force limited because of its greater capacity of pressure transfer, versus the conventional die-compaction process. Compared with die compaction, isostatic compaction tends to provide more uniform pressure distribution within a powder charge, with greater density uniformity in the resulting compact, essentially as a benefit of the absence of the die-wall friction which is associated with the mechanical pressing process. Consequently, isostatic compaction yields increased and more uniform density at a given compaction pressure. As a result of the available capacity of compaction pressure and uniform pressure distribution, the cross-section to height ratio is not a limiting feature in isostatic pressing as it is with mechanical die-compaction.
Another benefit of the isostatic process is the elimination of the die lubricants used in mechanical pressing. Typically these lubricants are mixed into the powder charge to facilitate compact ejection from the die and to avoid cold welding of the compact to the die wall, in the mechanical pressing. Absence of these mixed-in lubricants in isostatic processing permits higher pressed densities and eliminates problems associated with use of die lubricants, such as Potential contamination of the compact or the need for removal of the lubricant prior to sintering which can cause blistering.